Support Firestore Modular SDK through new Adapter and Serializer

[mikkopaderes]

Part of #178

This change introduces the cloud-firestore-modular adapter and serializer which makes uses of the Firestore Modular SDK. Originally, it was planned to avoid creating a new module as we could've combined Compat and Modular SDK code style as shown here. But the support for it is poor and not everything is compatible.

e.g.

import { writeBatch } from 'firebase/firestore';

...

const docRef = db.collection('users').doc('user_a');
const batch = writeBatch(db);

batch.set(docRef, { name: 'Foo' }); // <-- invalid because docRef uses compat SDK

So instead, it's now ending up to be an all-in transition towards Modular SDK and the Compat SDK adapter and serializer will now be in maintenance mode and will be dropped when Firebase v10 gets released. I recommend everyone to start transitioning to the cloud-firestore-modular path moving forward.